This protocol describes an optical method to directly stimulate a neuron (i.e., without using any caged chemicals or genetic probes) using an infrared ultrafast mode-locked laser. This method can trigger action potentials in a targeted neuron when a laser beam is applied to the somatic membrane. Alternatively, it can mimic excitatory postsynaptic potentials (EPSPs) when applied to dendritic spines. The protocol has been applied successfully using juvenile (postnatal day 7-14) C57 mouse neocortical and hippocampal acute slices (?300-µm thickness). It can be used in conjunction with slices bulk loaded with calcium indicators, such as Fura-2 AM.